1. Field of the Invention
This invention relates to medical imaging systems using nuclear magnetic resonance. In a primary application the invention relates to projection imaging of specific materials having unique NMR properties.
2. Description of Prior Art
Nuclear magnetic resonance, abbreviated NMR, represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, magnetic moments are excited at specific spin frequencies which are proportional to the local magnetic field. The radio frequency signals resulting from the decay of these spins are received using pick-up coils. By manipulating the magnetic fields, an array of signals are provided representing different regions of the volume. These are combined to produce a volumetric image of the density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1255. The basic concepts are described in the lead article, "Introduction to the Principles of NMR" by W. V. House, pp. 1220-1226.
A number of three-dimensional methods are described. One important one is described by P. V. Lauterbur and C. M. Lou entitled, "Zeugmatography by Reconstruction from Projections," pp. 1227-1231. In this approach, a linear field gradient is superimposed on the strong axial magnetic field. As a result of the gradient, each plane in the volume, in a direction normal to the gradient, experiences a different resonant frequency. A burst, containing a spectrum of frequencies, is used to simultaneously excite each of the planes. The received signal, following the excitation, is then Fourier transformed into its individual components. The amplitude at each frequency represents a planar integration of the proton density. This process can be repeated using a gradient field in different directions to collect information about arrays of planes. These planar integrals can be used to produce two-dimensional projection images of a volume or, alternatively, three-dimensional information about the proton density of each voxel in the volume.
The projection image is obtained by obtaining the integrated density of substantially all planes which are normal to the plane of the projection image. The total number of planes required, at all angles and positions, is substantially equal to the number of pixels in the two-dimensional projection image. The reconstruction procedure involves the classical reconstruction from projections widely used in current computerized tomography systems. The most generally used procedure is that of convolution-back projection.
The resultant two-dimensional projection images have a number of drawbacks and, as a result, are not used. Firstly, the superimposed intervening structures make it very difficult to visualize the desired structure, be it an organ or tumor. Secondly, the nature of this imaging procedure is such that all of the measurements affect every reconstructed pixel. This makes the image particularly sensitive to motion. Any motion of the object will cause artifacts in the image due to inconsistencies where the object does not match its projections. These artifacts can often obscure the desired information.
To avoid the problems of intervening structures, three-dimensional reconstructions are made which provides cross-sectional images. The approach taken in the Lauterbur paper involves making an array of two-dimensional projection images at every angle through the object. Lines in these projection images represent line integrals or projections of cross-sectional planes of the object. Thus, again using classical reconstruction techniques, any desired cross-sectional plane can be reconstructed. The intermediate two-dimensional projections are not used for the reasons discussed.
Although these cross-sectional images are free of intervening structures, they are unsuitable for many medical problems. The cross-sectional format is often difficult to interpret. In addition, the acquisition of three-dimensional data takes a relatively long time, thus resulting in a variety of artifacts due to the various physiological motions of the body.
A second general method of acquiring and processing NMR imaging data is described in a paper by E. R. Andrew entitled "Nuclear Magnetic Resonance Imaging: The Multiple Sensitive Point Method" pp. 1232 to 1238 of the same issue. In this method, a selective system is used which acquires data from individual voxels in the volume of interest. This is accomplished using dynamically varying fields for the gradients. In general, with these dynamic fields, all but the small region not containing the time-varying field integrates to zero. Thus, if time varying fields of different frequencies are applied to three orthogonal axes, only a single point or voxel will not be time-varying. The signal will therefore represent solely that point without requiring reconstruction from projections.
The difficulty with this system is that it requires a very long data acquisition time since the signal is taken from one voxel at a time. Sufficient time must be spent at each voxel to provide an adequate signal to noise ratio. This problem is alleviated by using dynamic gradients on two axes and a static gradient on the third axis. Thus, in the direction of the third axis, each position again corresponds to a different frequency. Using wideband excitation and Fourier transforming the received signal the frequency spectra simultaneously provide the density of an array of voxels along a line. The line is that corresponding to the intersection of the two orthogonal dynamic gradients where all but a single line averages to zero.
Although this method avoids the motion artifacts caused by reconstruction from projections, it continues to provide a relatively long data acquisition time with the resulting blurring from physiological motions including respiratory and cardiovascular. In addition it is a three-dimensional imaging system which provides cross-sectional images.
A third imaging method is also line or point selective and is described in a paper by L. E. Crooks entitled, "Selective Irradiation Line Scan Techniques for NMR Imaging" of pp. 1239-1244 of the same issue. This general approach has a number of variations. In one, a selective pulse is used to excite a single plane of interest using a static gradient and an appropriately shaped pulse. The resulting signal from the excited plane is stored. Following equilibrium an orthogonal plane is excited with a higher intensity such that the magnetization is inverted or made negative. Irradiation of this type produces no received signal. The first step is then repeated by selectively exciting the plane of interest and storing the resultant signal. In this case, however, a line in the plane of interest will be missing since it has been saturated by the high intensity excitation of a plane orthogonal to the plane of interest. Thus the line of intersection is not included in the resultant signal. A simple subtraction of the first and second stored signals represents the line of intersection. By measuring different lines at many angles and positions in the plane of interest, using this subtraction procedure, a reconstructed image of the plane is made using classical reconstruction from projection techniques.
An alternative approach using the same line intersection of orthogonal planes avoids the subtraction operation. In this case the orthogonal plane is immediately excited with inverting radiation. The line of intersection is affected so as to produce a spin echo signal at a later time. Thus, at this later time, the signal represents the desired line only. Again, an array of line integral signals are used to provide a cross-sectional image.
Similar sensitive point and sensitive line methods have been suggested which results in saturation of all but a specific plane of interest. This is immediately followed by a similar excitation in an orthogonal direction which saturates everything in the plane except a line. Either the line integral signal can be acquired, or a third orthogonal excitation can be used to acquire the signal from a point or voxel. Saturation is achieved by a relatively long "burn" radio frequency pulse, in the presence of a gradient, which demagnetizes the region corresponding to the frequencies excited. This procedure is described in a paper by A. N. Garroway, P. K. Grannell and P. Mansfield, "Image Formation in NMR by a Selective Irradiative Process," which appeared in J. Phys. C: Solid State Physics, Vol. 7, 1974, pp. L457-L462.
An additional approach to NMR imaging is described in a recent book entitled Nuclear Magnetic Resonance Imaging In Medicine, published in 1981 by Igaku-Shoin, Ltd., Tokyo. Chapter 3 of this book, by Lawrence E. Crooks, provides an overview of the various imaging techniques. In addition to those already mentioned there is another planar integration approach described on pp. 44-47. Here, each plane integral is phase encoded by applying a gradient normal to the plane. When the gradient is removed, the nuclei along the plane have cyclical phase distributions, depending on the strength of the magnetic field. By acquiring these planar integrals using phase distributions with different spatial frequencies, information is acquired about each line in the plane. This information is decoded again using Fourier transforms. This approach has been termed spin warp imaging.
Each of the data acquisition systems described can be used to measure density, the longitudinal relaxation time T.sub.1 and the spin-spin relaxation time T.sub.2. As described in the previously referenced book, Nuclear Magnetic Resonance Imaging in Medicine, the density information can be acquired using an excitation which rotates the magnetic moment by 90.degree., and measuring the free induction decay or FID signal. T.sub.1 can be measured by inverting the magnetic moment with a 180.degree. excitation, and the following it with a 90.degree. excitation whereby the resultant signal will be determined by the amount of decay. Alternatively, two 90.degree. excitations, separated by a time less than 4T.sub.1, will result in signals whose amplitude differences can be used to determine T.sub.1. The decay time of the FID signal cannot directly be used to measure T.sub.2 since the inhomogeneity of the fields cause a rapid decay. However, if 180.degree. inversion excitations are periodically applied, these serve to cancel the effects of the field inhomogeneity. If the amplitudes of the spin echoes following these inversion excitations are observed and compared to the initial FID signal following the 90.degree. excitation, the decay is indicative of T.sub.2. A variety of equivalent methods have been described for the measuring of the components. Cross-sectional images have been made of each of these components.
This same book, Nuclear Magnetic Resonance Imaging in Medicine, describes an alternate system of radio frequency excitation which has the advantage of being relatively noncritical as to the uniformity of the radio frequency excitation field. This is known as "adiabatic fast passage" or AFP. Here the frequency of the excitation signal is swept through resonance to provide a 180.degree. inversion excitation, and swept over half that interval, to provide a 90.degree. excitation. Its use is desirable where accurate excitations are needed without requiring a high degree of uniformity of the rf field.
Thusfar images have been made essentially of hydrogen, with its single proton nucleus. Other elements and isotopes have insufficient activity to produce cross-sectional images in a reasonable time. These elements have been studied, however, in non-imaging situations where the material content of a volume of interest is evaluated. It would be highly desirable, however, to provide images of other materials for a variety of applications including the study of metabolism.
An additional image sequence, which is used in some of the present commercial instruments, is described in a paper by I. R. Young, et al., entitled "Magnetic Resonance Properties of Hydrogen: Imaging of the Posterior Fossa," Amer. J. of Radiology, Vol. 137, pp. 895-901, November 1981. Here a single excitation burst is used to select the desired plane. This burst takes place in the presence of a z gradient, so that the burst frequency selects a specific xy plane in the volume. Immediately following the burst, when the FID signal is being received, the z gradient is turned off and a transverse gradient applied. This results in each line in the plane, normal to the transverse gradient, generating a different frequency. For cross-sectional imaging this sequence is repeated with the transverse gradient rotated to different angles so as to provide a complete set of projections. The reconstruction is accomplished by classical methods of reconstruction from projections, such as filtered back projection.
This application represents improved versions of the basic NMR selective projection imaging as described in U.S. Pat. No. 4,486,708 by the same inventor. In this application a unique system of projection NMR imaging is shown whereby projection measurements are made of a plurality of NMR components. There are then combined to obtain isolated projection images of specific materials. As to specifics, the U.S. Pat. No. 4,486,708 discusses the use of .rho., T.sub.1, T.sub.2, velocity, and the resonant frequencies of different materials. However, to cancel all of the undesired specific materials in the body and retain only a material of interest, a large number of independent NMR components are desired.
U.S. Pat. No. 4,486,708 also disclosed a unique approach to projection imaging whereby projections are obtained over a selected region of the volume. In one approach the non-selected regions are insensitized prior to making the sequence of projection measurements. However, during the time of making these measurements, some recovery of the non-selected regions can occur, making the region selectivity incomplete.
Although projection imaging has inherent basic advantages as far as signal-to-noise ratio, as discussed in U.S. Pat. No. 4,486,708 it does have the disadvantage that signals are received only once from each region. This factor tends to diminish the SNR relative to the imaging modalities.
Projection imaging of the density can take place relatively rapidly since each successive plane can be excited in sequence, with the resultant FID signal received. This is in contrast to cross-sectional imaging where the same planar section is repeatedly excited, with time allocated for relaxation between excitations, thus significantly increasing the acquisition time. When, however, projection images are obtained of signals sensitive to the relaxation times, as described in U.S. Pat. No. 4,486,708, each projection involves a time comparable to the relaxation time constant. Thus the acquisition time of this NMR component can be relatively long in projection imaging.